Solution structure of the chicken skeletal muscle troponin complex via small-angle neutron and X-ray scattering

Troponin is a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle by participating in a series of conformational events within the actin-based thin filament. Troponin is a heterotrimeric complex consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT). Ternary troponin complexes have been produced by assembling recombinant chicken skeletal muscle TnC, TnI and the C-terminal portion of TnT known as TnT2. A full set of small-angle neutron scattering data has been collected from TnC-TnI-TnT2 ternary complexes, in which all possible combinations of the subunits have been deuterated, in both the +Ca2+ and -Ca2+ states. Small-angle X-ray scattering data were also collected from the same troponin TnC-TnI-TnT2 complex. Guinier analysis shows that the complex is monomeric in solution and that there is a large change in the radius of gyration of TnI when it goes from the +Ca2+ to the -Ca2+ state. Starting with a model based on the human cardiac troponin crystal structure, a rigid-body Monte Carlo optimization procedure was used to yield models of chicken skeletal muscle troponin, in solution, in the presence and in the absence of regulatory calcium. The optimization was carried out simultaneously against all of the scattering data sets. The optimized models show significant differences when compared to the cardiac troponin crystal structure in the +Ca2+ state and provide a structural model for the switch between +Ca2+ and -Ca2+ states. A key feature is that TnC adopts a dumbbell conformation in both the +Ca2+ and -Ca2+ states. More importantly, the data for the -Ca2+ state suggest a long extension of the troponin IT arm, consisting mainly of TnI. Thus, the troponin complex undergoes a large structural change triggered by Ca2+ binding.